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Comparison of Natural Gases Produced from Upper Cretaceous Fruitland Formation Coal Beds and Adjacent Reservoirs, San Juan Basin, New Mexico and Colorado

Dudley D. Rice, Charles N. Threlkeld

Significant quantities of natural gas are being produced from coal beds of the Upper Cretaceous Fruitland Formation and from adjacent sandstone reservoirs. The coal rank ranges from high-volatile bituminous A to medium-volatile bituminous (Ro values range from 0.7 to 1.4%). On the basis of chemical and isotopic composition and associated coal rank, the gases are interpreted to be thermogenic. Gases from the coal beds show little isotopic variation (^dgr13C1 values range from -43.7 to -42.0 ^pmil), are chemically dry (C1/C1-5 values range from 0.98 to 0.99), and contain significant amounts of CO2 (as much as 6%). These gases are interpreted to result from devolatilization of the coal, which yields methane, C 2, and water as the primary products. In contrast, gases from sandstones in the overlying Fruitland Formation and underlying Pictured Cliffs Sandstone are isotopically heavier (^dgr13C1 values range from - 43.9 ^pmil to - 38.5 ^pmil), are chemically wetter (C1/C1-5 values range from 0.86 to 0.94), and contain less CO2 (< 2%). Minor amounts of waxy, immature oil are also produced. These gases are thought to derive from nonmarine kerogen (Type III) dispersed in the underlying marine Lewis Shale and the main part of the Fruitland Formation. These two gas types are distinctly different from associated gases produced from the underlying Upper Cretaceous Dakota Sandstone and Tocito Sandstone Lentil, which are interpreted as p oducts of marine kerogen (Type II).

A few of the gas samples from sandstones in the Fruitland Formation and Pictured Cliffs Sandstone are almost identical in chemical and isotopic composition to those of coal beds in the immediate area. These are probably migrated from nearly coal beds.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.